1. Field Of The Invention
The present invention relates to computer input devices such as digitizers referred to as a touchpad for the control of a cursor upon a display screen of a computer system, and more particularly to the systems and methods necessary to acquire signals from such input devices and to convert the acquired signals to digital codes necessary to control a cursor upon a display screen.
2. Description of Related Art
Touchpads are small digitizer based input devices that are used to replace a computer input device commonly referred to a mouse. The touchpad digitizers may be of three types, capacitive, resistive and electromagnetic.
Referring to FIG. 1, the surface 12 of the touchpad becomes a "writing surface" for capturing the position of an pointed object 10 such as a finger, pen or stylus upon the touchpad. The touchpad signals are analog signals that will be captured by a touchpad interface circuit 28 and translated to digital codes that will be transferred to a computer system 32 on an interface 30. The interface 30 may be an industry standard serial interface, an industry standard parallel interface, or a custom interface requiring special adapter circuitry within the computer system 32 to accept the digital codes from the touchpad interface 28.
An example of a resistive touchpad, as shown in FIG. 1, is made up of multiple layers of resistive films and protective layers. The protective hard coating 12 is the surface onto which the pointed object 10 is pressed upon. A first layer of resistive film 14 is attached to the protective hard coating 12 on the surface opposite the writing surface. This first layer of resistive film forms the Y-plane of the touchpad. Attached to the surface of the Y-plane resistive film 14 opposite the surface attached to the hard protective coating 12 is a second resistive film 16. This second resistive film 16 forms the X-plane of the touchpad. Finally attached to the side of the X-plane resistive film 16 is a supporting back layer 18. This back layer provides protection and mechanical support for the for the X-plane and Y-plane resistive films 14 and 16.
The touchpad interface 28 is connected through the touchpad interface lines 20, 22, 24, and 26. Each line will provide a stimulus such as a current or voltage to the periphery of the X-plane resistive film 16 and the Y-plane resistive film 14. As shown in FIG. 2, as the pointed object 10 is pressed 40 on the touchpad surface 12, the Y-plane resistive film 12 will deform and touch the X-plane resistive film 14. The X-plane resistive film can not deform because it is supported by the supporting back layer 18. This causes the Y-plane resistive film 14 and the X-plane resistive film 16 to come into contact with each other. This will cause a response in the form a change in voltage or current depending upon whether the stimulus from the touchpad interface 28 of FIG. 1 is a constant voltage or a constant current. If the stimulus from the touchpad interface 28 of FIG. 1 is a constant voltage the currents through the touchpad interface lines 20, 22, 24, and 26 will be modified according the position of the pointed object 10 on the touchpad surface 12. However, if the stimulus from the touchpad interface 28 of FIG. 1 is a constant current the voltages between the touchpad interface lines 20, 22, 24, and 26 will be modified according the position of the pointed object 10 on the touchpad surface 12.
Within the touchpad interface 28 of FIG. 1 will be a touchpad driver. The drivers will consist of metal oxide semiconductor transistors (MOST) M1, M2, M3, and M4 that will selectively couple the power supply voltage source Vdd and the ground reference point GND to the X-plane 16 and the Y-plane 14. When the pointed object 10 is pressed on the touchpad 110 at point 40, the driver select lines X+sel and X-sel will be activated applying the power supply voltage source Vdd to one side on the X-plane at the Vx+ and the ground reference point GND to the opposite side of the X-plane at the Vx-.
The parasitic resistances Rpx+ and Rpx- represent the resistance of the distribution network respectively from point X+ to the side of the X-plane at Vx+ and the on resistance of the MOST M1 and the from the point X- to the opposite side of the X-plane at Vx- and the on-resistance of the MOST M2.
The resistors Rpx+, R.sub.xplane, and Rpx- form a voltage divider network. The pointed object 10 for the Y-plane 14 to contact X-plane 16 at point Vx. The point Y- will be the sense point for the voltage Vx developed at the contact point.
After the select lines X+ sel and X- sel are deactivated, the select lines Y+ sel and Y- sel are activated, connecting the power supply voltage source Vdd to the Y-plane at point Vy+ and the ground reference point GND to the Y-plane at point Vy-.
As described above, parasitic resistors Rpy+ and Rpy- will be formed as a result of the distribution network and the on-resistances of the MOST M3 and M4.
The resistors Rpy+, R.sub.yplane, and Rpy- form a voltage divider network. The pointed object 10 has forced the Y-plane 14 to contact the X-plane 16 at point Vx. Now the point X- will sense the voltage developed at the point Vx.
Referring back to FIG. 1, the touchpad interface 28 will have a set of analog to digital converters that will sense the change in the analog responses from the touchpad interface lines 20, 22, 24, and 26 and convert them to digital codes indicating the absolute position of the pointed object 10 upon the touchpad surface 12. For the computer system 32 to use the absolute coordinates generated by the touchpad interface 28 to control the movement of the cursor 36 upon the display screen 34, these absolute coordinates must be modified to codes that define the relative motion of the cursor 36. The relative motion will be the speed and direction of the cursor 36 as it is moved across the display screen 34. The modification from absolute coordinates to relative motion information must be done within an internal mouse emulation program resident within the computer system 32 or the touchpad interface 28.
If the touchpad is to emulate the mouse movements, when the cursor 36 is to be moved across the display screen 34 for a relatively long distance, the pointed object 10 must be lifted and placed back on the touchpad surface 12 repeatedly giving a "rowing" motion to get the cursor 36 to move the long distance. In traditional mouse operations, if the cursor 36 is to drag an object being displayed upon the display screen 34 a button on the mouse is depressed while the mouse is moved. The button can be held depressed while the mouse is moved in the rowing motion to drag the object across the display screen 34. This is difficult to accomplish on the touchpad. If the pointed object 10 is lifted from the touchpad, the touchpad interface 28 will not be able to communicate the "rowing" motion to indicate that the cursor 36 is to travel a long distance. Also, the touchpad interface 28 will not be able to communicate that there is an intention for the cursor 36 to drag the object on the display screen 34. Additional buttons must be added to the touchpad or special areas within the surface of the touchpad surface 12 in order for the touchpad interface 28 to communicate the desire for the cursor 36 to be moved long distances across the display screen 34 or that the cursor 36 is to drag objects upon the display screen 34.
U.S. Pat. No. 5,327,161 (Logan, et al.) use a method to emulate mouse input devices using a program resident within a computer system. A touchpad input device has a controller that generates a digital code that contains the absolute position of a pen or finger on the mouse pad. This requires a special interface that is unique to the touchpad circuitry. Additionally, this patent describes a method for the continuation of cursor movement when a pointed object is touching the touchpad and has been moved on the touchpad to a special border area. The pointed object must be stopped within the border for the continuous motion to be engaged. The direction of the scrolling may be made as a modification of the original direction and velocity of the pen prior to the transiting and stopping within the border area of the touchpad. This modification will be made as a change in the velocity of the movement of the cursor along an axis parallel to the edge of the touchpad adjacent to the border area where the pointed object is resting.
U.S. Pat. No. 5,376,946 (Mikan) describes a circuit using an EPROM to convert signals from a touch screen adhered to a computer display screen to digital codes of the industry standard computer input mouse protocols.
U.S. Pat. No. 5,266,750 Yatsuzuka) discloses a tablet input device and circuitry for providing stimulating voltages to the tablet input device and for sensing the response voltages from the tablet input device when the tablet input device is being pressed. The circuitry provides an OFF state wherein power to the tablet is minimized during a waiting period.
U.S. Pat. No. 5,543,590 (Gillespie, et al.) describes a sensor system that can detect the location of a finger or stylus on a sensor matrix. The location is determined and translated as electrical signals for use in other circuitry such as a computer system to control a cursor upon a display screen. Further this patent discusses an "edge motion" detection feature that will allow a finger or stylus within a "outer zone" of the sensor matrix to move the cursor across a display screen for long distances and avoid the "rowing" motion.
U.S. Pat. No. 5,543,591 (Gillespie, et al.) discloses methods for recognizing tapping, pushing, hopping and zigzagging gestures upon a conductive sensor pad that can be interpreted into cursor control motions such as clicking, double clicking, and click and drag use with computer mouse devices. Further this patent also describes the "edge motion" feature as described in U.S. Pat. No. 5,543,590 (Gillespie, et al.).